The invention spawned from research for economical processes for the conversion of low valued carbohydrates to higher valued products other than ethanol. Low valued carbohydrates, in many cases considered annoying waste products today, come from a number of sources that include molasses, food production wastes, wood or cellulosic wastes, etc.
The research first addressed the production of esters of acrylic acid, and particularly production of alkyl acrylate esters from black-strap molasses, a large volume waste product of the sugar cane industry. The first processes involved dehydration of lactate esters (i.e. removal of a hydroxyl group and hydrogen and forming a water molecule) to form acrylate esters, and involved fermentation technology for production of the lactate esters.
The acrylate products have a firm position in the marketplace for industrial coatings, water based emulsion-type paints for interior as well as exterior applications, and for plastic sheeting and films. The production of acrylates is a mature industry that has involved the petrochemistry of materials such as acetylene, ethylene cyanhydrin, acrylonitrile and propylene. Processes based on the oxidation of propylene are dominant today. Propylene is a coproduct for some major processes for the production of ethylene. However, some important ethylene processes yield no propylene at all. Furthermore, there are no major production processes that are operated to produce propylene as the major product. Thus, propylene varies widely in its price and availability. Its inherent close alignment with the petrochemical industry generally, and with ethylene in particular, creates rather uncertain and unstable economics.
Lactic acid is now produced commercially in the United States essentially only from acetaldehyde and hydrogen cyanide. On the other hand, there is much low cost carbohydrate material that is anticipated to assure a stable and long term supply if efficient, economic processes could be developed for converting this material into lactic acid, esters of lactic acid, acrylic acid, or esters of acrylic acid. Such processes will most likely entail the fermenting of the carbohydrate material with a lactic acid forming organism. Up to this time, essentially all commercial processes of fermentation to produce lactic acid have involved calcium carbonate as the means for maintaining pH at high enough levels to maintain the continuous activity of the fermentation organism. However the resulting salt, calcium lactate, has a limited solubility. To assure that the fermentation liquor is maintained as a pumpable fluid, the presence of a large fraction of solid hydrated calcium lactate must be avoided. This necessitates overall fermentation operations on rather dilute levels.
U.S. Pat. No. 2,565,487 to Filachione et al. and an article entitled "Lactate Esters by Reaction of Ammonium Lactate With Alcohols," by Filachione et al. disclose the use of NH.sub.3 in a fermentation process to produce ammonium lactate, with subsequent conversion to lactate esters. These processes use NH.sub.3 for pH control. The ammonium lactate which is produced is subsequently converted to an ester by prolonged heating in the presence of a large excess of butanol or higher alcohol at atmospheric pressure. Reaction times are stated to take from 5 to 10 hours, with as much as 17 hours being required. Even then, the percentage yields are not very great.
Furthermore, although the Filachione et al. patent states in one instance that methyl or ethyl alcohols are useable, they also state that the higher boiling alcohols are more suitable to the reaction than lower boiling alcohols. It would be advantageous to develop a process that could utilize either high boiling or low boiling alcohols for conversion of ammonium lactate to a lactic acid ester. It would further be preferable to develop a process which requires a significantly reduced reaction time, and results in higher conversion to the lactate ester.
The production of an acrylic acid ester from a lactate ester would at first glance seem to require simply removing a hydroxyl group and hydrogen atom from adjacent carbon atoms to result in formation of the acrylic acid structure. However, other reactions are dominant and largely inhibit this conversion to an acrylate. The principal competing reaction is formation of self reaction products, such as lactides, which are subsequently more readily decomposed into fragments such as carbon monoxide, acetaldehyde, and water.
Prior art techniques to avoid this involve formation of intermediates which impair lactide production, and by the use of certain catalysts to promote the direct removal of H and OH from the lactate ester. For example, U.S. Pat. No. 2,859,240 to Holmen discloses a number of catalysts useful in a process conducted at between 250.degree. C. to 550.degree. C. to produce the acrylate.
Aspects of the following invention would also be useful in the production of purified lactic acid. Lactic acid may be a suitable feed material for the production of biodegradable plastics. The polystyrene and polyethylene plastics of this day have wide use because of their low cost and desirable properties. However, they create horrendous disposal problems. Processes for the production of biodegradable lactic acid polymers are being developed, but the cost of producing a high quality commercial grade of lactic acid may make the resulting plastic prohibitively expensive. This problem could be overcome by an efficient, low cost production of lactic acid from low-cost carbohydrate materials.